[Chapel Merge] Bump up sphinxcontrib version to 0.0.20

Branch: refs/heads/master
Revision: 1108202
Author: e-kayrakli
Log Message:

Merge pull request #17382 from e-kayrakli/sphinxcontrib-0.0.20

Bump up sphinxcontrib version to 0.0.20

The new version has the new keywords we are adding in Chapel 1.24, as well as
the support for the new : operator.

[Reviewed by @lydia-duncan]

Modified Files:
M third-party/chpl-venv/chpldoc-requirements.txt

Compare: https://github.com/chapel-lang/chapel/compare/5e5f859fe7ed...1108202e645d